Adopt memorystore for tests

[hanzei]

Summary

This PR adopts the MemoryStore (mattermost/mattermost#26244) from the tests in server/plugin/subscriptions_test.go and removes the mocks.

I've included a cleanup of p.API usage via 3f0f4fc in this PR.

Ticket Link

None

[hanzei]

@mickmister @raghavaggarwal2308 Gentle reminder to review the PR

[raghavaggarwal2308]

@hanzei LGTM 👍🏽

Note: tests are failing in CI

[hanzei]

I've reached out to @ayusht2810 regarding the failing CI in https://community.mattermost.com/core/pl/z7833z1f6iyaibmib4af13fzar

[raghavaggarwal2308]

I've reached out to @ayusht2810 regarding the failing CI in https://community.mattermost.com/core/pl/z7833z1f6iyaibmib4af13fzar

@hanzei The tests are working fine on my local for master and your branch as well. Can you please try re-running the CI here?

[hanzei]

@raghavaggarwal2308 I've re-triggered CI, which fixed the issue.

Please note that the tests on master also fail. It seems like we have a flaky test there.

[hanzei]

Ok, master is also green now. Seems like the issue is fixed 🤷

[hanzei]

@raghavaggarwal2308 @mickmister Do you think adopting the memory store would be helpful for other plugins that extensively make use of the KV store like Gitlab or JIRA?

[hanzei]

@raghavaggarwal2308 The nightly run failed again: https://github.com/mattermost/mattermost-plugin-github/actions/runs/8164766247/job/22320677668

[Kshitij-Katiyar]

@raghavaggarwal2308 The nightly run failed again: https://github.com/mattermost/mattermost-plugin-github/actions/runs/8164766247/job/22320677668

@hanzei I tried running the commands locally many times but was not really able to reproduce this.

[hanzei]

My own tests were the ones that were flaky. 🤦 Submitted mattermost/mattermost-plugin-starter-template#197.